It's just a quest in Ashenvale. A Shivera (who, remember, are the holy people of the Burning Legion, not small time demons) captured a few night elves to trap the player, you beat her, and the only way to free the trap is to give the Night Elves the heart to eat. The night elves are the ones who say they have to eat it to break the trap, so it's obviously something they've seen before.